The present invention relates to a photovoltaic element for use in a solar cell, an optical sensor, etc. having a structure comprising a transparent conductive film deposited on a semiconductor layer, like a pin junction, and also relates to a method for manufacturing the photovoltaic element.
In recent years, the introduction of a photovoltaic power generation system that is installed on the roof of a house has been promoted rapidly. A solar cell for use in the photovoltaic power generation system utilizes, for example, a photovoltaic element which is manufactured by depositing i-type and p-type amorphous or microcrystalline semiconductor layers and a transparent conductive film made of a material such as an indium tin oxide (ITO) and zinc oxide (ZnO) successively on an n-type crystalline silicon wafer and then forming a collecting electrode on the transparent conductive film.
Since a solar cell using such a photovoltaic element is generally mounted outdoors, it needs to have high environment resistance reliability. Hence, conventionally, when a photovoltaic element is incorporated as a product into a module, a cover glass is often used for the purpose of protecting the photovoltaic element, and thereby ensuring environmental resistance of the module.
However, since, in general, inexpensive white plate glass is used as the cover glass, alkaline ions such as Na, Li and K contained in the white plate glass are sometimes diffused into the amorphous or microcrystalline semiconductor layers through the transparent conductive film under conditions such as a high humidity condition and exert a vicious influence. For example, when alkaline ions are diffused into the amorphous or microcrystalline semiconductor layers, this diffusion causes a diffusion potential change and may cause the problem of deterioration in the characteristics of the photovoltaic element.
It is therefore desirable that the photovoltaic element itself should have excellent environment resistance, particularly, good resistance to alkaline ions, and there has been a demand for an improved transparent conductive film. Moreover, in order to achieve high efficiency, the transparent conductive film of the photovoltaic element is required to have high light transmittance and low electrical resistance. In general, in order to realize high light transmittance and low electrical resistance, it is necessary to improve the crystallinity of the transparent conductive film. However, when ITO and ZnO which are polycrystalline substances are used, the crystal grains become larger, and accordingly the influence of the crystal grain boundaries increases. Thus, there are possibilities of promotion of diffusion of alkaline ions through the grain boundaries as a path and a decrease in the environment resistance reliability.